Certain polyoxypropylene/polyoxyethylene (POP/POE) copolymers, called poloxamers, have beneficial biological effects when administered to a human or animal. These copolymers have been used for treating circulatory diseases either alone or in combination with other compounds, including, for example, anticoagulants, free radical scavengers, anti-inflammatory agents, antibiotics, membrane stabilizers and perfusion media. Poloxamer 188 (P188) (see, e.g., U.S. Pat. No. 5,696,298) is useful for treating pathologic hydrophobic interactions in blood and other biological fluids of humans and animals. Commercially available preparations of poloxamers contain highly heterogeneous populations of molecules that vary widely in the size and configuration of the constituent molecules. The diversity of molecules present in the commercially available poloxamers can make prediction of the biological activity difficult and lead to unwanted biological activities. Hence, there is a need for alternative preparations of poloxamers.